I've been researching some of the slightly earlier militias and they seem to vary enormously. The Khyber Rifles, for example, were uniformed and looked pretty much like regular troops, but other militias seemed to wear their own tribal clothing. The other question is what you want the guys to be armed with.
Foundry make some lovelly tribal types, but they are armed with muzzle loaders. Foundry also make nice Sikhs/Guides (the latter are simple conversions of the former) armed with Martinis, but there aren't many poses (and also do nice Sikhs in the Darkest Africa range). The Perrys make Sikh and Bengal infantry, armed with Sniders, in their Sudan range. Copplestone do Indian infantry in the BoB range armed with Lee-Enfields and Woodbine do WW1 Indian infantry. Some modern Afghan ranges (like the Mongrel range) include a few Lee-Enfield armed figures that you might want to consider.
A top quality company will be doing some very suitable tribal types in the very near future, but I'll let them announce it if they're reading (so you may want to hold on for a few months).
All of the above are in 28mm. I've no idea what might be out there in 20mm.
Do you have photos of they guys that you want to model?